1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a non-volatile memory cell and a method for fabricating the same.
2. Description of the Related Art
Non-volatile memory devices that use charge-trapping mechanisms have been widely studied. An earlier trapping-type non-volatile memory device is the mirror-bit SONOS memory as described in U.S. Pat. No. 5,768,192, which uses a nitride layer disposed between two oxide layers for charge storage. Since silicon nitride is an insulating material and the charges trapped in the nitride trapping layer are localized, two bits can be stored in one memory cell as a hot charge injection mechanism used for programming.
Recently, a category of trapping-type non-volatile memory that uses isolated conductive nano-particles for charge storage has been proposed. For example, U.S. Pat. No. 6,342,716 B1 and U.S. Pat. No. 6,413,819 disclose a trapping-type non-volatile memory that uses isolated dot elements for charge storage. U.S. Pat. No. 6,165,842 discloses a method that forms silicon nano-crystals for charge storage by etching a polysilicon layer. U.S. Pat. No. 6,297,095 B1 discloses a method that forms silicon nano-crystals for charge storage with controlled LPCVD or UHVCVD. Besides, U.S. Pat. No. 6,303,516 B1 discloses a method that forms fine metal particles for charge storage by using an antigen-antibody pair binding method. In addition, U.S. Pat. Nos. 5,783,263 and 5,876,480 disclose other methods for making nano-crystals. U.S. Pat. No. 5,783,263 discloses a method that forms metal nano-particles by using carbon arc discharge on a graphite rod packed with the same metal, alloy, or oxide of metal or alloy. U.S. Pat. No. 5,876,480 discloses a method that forms metal nano-particles by reducing ions of the same metal with some catalytic metal ions that are carried by vesicles in a dispersion system.
To well control the electrical properties of a trapping-type non-volatile memory that uses nano-particles for charge storage, good uniformity in density of the nano-particles is required. Unfortunately, the nano-particles made with any one of the aforementioned methods in the prior art have poor uniformity, and the electrical properties of the non-volatile memory cannot be well controlled.